Silicone rubber composition, silicone rubber sponge composition, and silicone rubber-covered wire

ABSTRACT

A silicone rubber composition comprising (A) an organopolysiloxane, (B) an inorganic filler, typically fumed silica, and (C) a specific halogen-free organic peroxide, typically 1,6-bis(p-toluoylperoxycarbonyloxy)hexane or 1,6-bis(benzoylperoxycarbonyloxy)hexane is safe and lends itself to atmospheric hot air vulcanization so that it can be extrusion vulcanized into silicone rubber having satisfactory physical properties without voids or surface tack. The silicone rubber composition is extrusion molded around an electrical wire, constructing a covered wire.

[0001] This invention relates to a silicone rubber composition of theorganic peroxide curing type, especially such a silicone rubbercomposition suited for wire coating, a silicone rubber spongecomposition, and a silicone rubber-covered wire.

BACKGROUND OF THE INVENTION

[0002] In the prior art, silicone rubber compositions of the organicperoxide curing type use organic peroxides such asbis(2,4-dichlorobenzoyl)peroxide, 2,5-di(t-butylperoxy)hexane anddicumyl peroxide as the curing agent. An appropriate type and amount ofcuring agent is selected and used depending on the molding technique ofa silicone rubber composition as well as molding temperature and otherconditions, and the type of silicone rubber.

[0003] Of these organic peroxide curing agents,bis(2,4-dichlorobenzoyl)peroxide is a superior curing agent and widelyused in the industry. This is because bis(2,4-dichlorobenzoyl)peroxideis stable at room temperature, achieves the fastest vulcanization ascompared with other curing agents and offers high productivity. As thesignificant feature, this curing agent is most effective underatmospheric hot air vulcanization (HAV) conditions.

[0004] However, bis(2,4-dichlorobenzoyl)peroxide produces chlorineby-products when thermally decomposed. The molding time is undesirablyprolonged because long-term heat treatment is needed to remove thechlorine by-products. It is believed from the environmental standpointthat the use of peroxide curing agents which will form such chlorineby-products becomes difficult or inhibited.

[0005] From such a standpoint, there is a need for a peroxide curingagent substitute for bis(2,4-dichlorobenzoyl)peroxide. JP-A 62-185750discloses bis(p-methylbenzoyl)peroxide as the halogen-free peroxidecuring agent. JP-A 63-130663 discloses1,3-bis(t-butylperoxy-carbonyloxy)-2,2-dimethylpropane as a halogen-freeperoxide curing agent. These peroxide curing agents have highdecomposition temperatures and suffer from the drawbacks of a slowvulcanization rate and poor productivity.

[0006] For the vulcanization of wire coating silicone rubbercompositions, various techniques are used depending on the type ofsilicone rubber composition and the physical properties required for thecured rubber. In general, heat treatment in the presence of organicperoxides is widely used. Such organic peroxides include benzoylperoxide, bis(p-chlorobenzoyl)peroxide,bis(2,4-dichlorobenzoyl)peroxide, dicumyl peroxide, di-t-butyl peroxide,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, t-butyl perbenzoate, andt-butyl cumyl peroxide. As the vulcanizing agents which ensure that costeffective silicone rubber-covered wires having good properties aremanufactured through extrusion hot air vulcanization,bis(2,4-dichlorobenzoyl)peroxide and other halogen-containing peroxidesare customary. However, when halogen-containing organic peroxides areused, decomposed products thereof are left in molded parts after curing,which will bleed out on the surface with the lapse of time. Because oftheir own poisoning effect, long-term post-curing is necessary.

[0007] To solve these problems, for example, JP-A 59-18758 proposes theuse of bis(o-methylbenzoyl)peroxide as the curing agent in siliconerubber compositions. The use of bis(p-methylbenzoyl)peroxide is alsoproposed as the curing agent in silicone rubber compositions. Thesecuring agents in the form of methyl-substituted benzoyl peroxide have ahigh decomposition temperature and a low vulcanization rate so thatsilicone rubber compositions using such curing agents tend to generatevoids upon curing. As a consequence, silicone rubber molded parts oftenhave varying physical properties, and coated wires undergo a lowering ofbreakdown voltage such as spark-over.

[0008] Meanwhile, silicone rubber sponge has physical propertiesinherent to silicone rubber as well as excellent properties includingheat resistance, freeze resistance, electrical insulation, flameretardance and compression set. Basically, silicone rubber sponge ismanufactured by combining a heat-curable silicone rubber compositionwith a curing agent and a blowing agent, and heating the composition forblowing and curing, thereby forming a sponge. In this process, a blowingability, a uniform fine cell structure, and a skin layer having a smoothtack-free surface are important, and the physical properties inherent tosilicone rubber must be retained.

[0009] With respect to the molding method, the compositions are curedand expanded in atmospheric hot air so that continuous molding ispossible. Organic peroxides enabling atmospheric hot air vulcanizationare generally halogen-containing peroxides such asbis(2,4-dichlorobenzoyl)-peroxide. When halogen-containing organicperoxides are used, there arise the problems that decomposed productsleft in molded parts after curing will bleed out on the surface with thelapse of time, and long-term post-curing is necessary because of thedecomposed products' own poisoning effect as previously mentioned.

[0010] Halogen-free benzoyl peroxide allows for atmospheric hot airvulcanization, but fails to provide satisfactory sponge properties withrespect to expansion and cell structure.

[0011] Then JP-A 10-182972 proposes the use of alkyl-substituted benzoylperoxides as a halogen-free curing agent. These peroxides have a highdecomposition temperature and a low vulcanization rate, leading to lowproductivity, and fail to provide satisfactory sponge properties withrespect to expansion and cell structure.

SUMMARY OF THE INVENTION

[0012] Therefore, an object of the invention is to provide a siliconerubber composition, especially for electrical wire coating, which has ahigh vulcanization rate and hygienic safety, and can be extrusionvulcanized in a continuous atmospheric hot air vulcanization manner intoa silicone rubber part without voids or surface tack. Another object isto provide a silicone rubber-covered wire using the silicone rubbercomposition and having improved withstand voltage properties.

[0013] A further object of the invention is to provide a silicone rubbersponge composition which can be cured with halogen-free organicperoxides without detracting from working efficiency and has a reducedpost-cure time, as compared with halogen-containing organic peroxides,and which is effectively expandable into a silicone rubber sponge havinga uniform fine cell structure and a skin layer with a smooth, tack-freesurface.

[0014] It has been found that when organic peroxides of the followingstructural formula (2), which are halogen free, are used as the curingagent in silicone rubber compositions, these organic peroxides create noenvironmental problem, have a high vulcanization rate satisfactory forproductivity, and function well under atmospheric hot air vulcanizationconditions. These organic peroxides are thus effective for producingsilicone rubber or silicone rubber sponge with satisfactory physicalproperties. When wires are covered with the silicone rubbercompositions, extrusion vulcanization can be effected without givingrise to such problems as voids and surface tackiness, and the resultingsilicone rubber coating has improved withstand voltage properties. Theinvention is predicated on these findings.

[0015] According to the invention, there is provided a silicone rubbercomposition comprising (A) an organopolysiloxane of the averagecompositional formula (1), (B) an inorganic filler, and (C) an organicperoxide of the structural formula (2).

R¹ _(a)SiO_((4−a)/2)  (1)

[0016] Herein R¹ is a substituted or unsubstituted monovalenthydrocarbon group and “a” is a positive number of 1.8 to 2.3.

[0017] Herein R² is independently hydrogen or alkyl, R³ is alkylene, andn is an integer of 1 to 3.

[0018] Also provided herein is a silicone rubber-covered wire comprisinga wire around which the silicone rubber composition has been extrusionmolded.

[0019] The invention also provides a silicone rubber sponge compositionfurther comprising (E) an organic blowing agent in addition to the abovecomponents.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] Component (A) of the silicone rubber composition according to theinvention is an organopolysiloxane of the average compositional formula(1).

R¹ _(a)SiO_((4−a)/2)  (1)

[0021] In formula (1), R¹ which may be the same or different is asubstituted or unsubstituted monovalent hydrocarbon group, preferably of1 to 12 carbon atoms, more preferably 1 to 8 carbon atoms, for example,alkyl groups such as methyl, ethyl, propyl and butyl, cycloalkyl groupssuch as cyclohexyl, alkenyl groups such as vinyl, allyl, butenyl andhexenyl, aryl groups such as phenyl and tolyl, aralkyl groups such asbenzyl and β-phenylpropyl, and substituted ones of the foregoing groupsin which some or all of the hydrogen atoms attached to carbon atoms arereplaced by halogen atoms, cyano groups or the like, such aschloromethyl, trifluoropropyl and cyanoethyl. The letter “a” is apositive number of 1.8 to 2.3, preferably 1.95 to 2.05. Theorganopolysiloxane should preferably have at least two alkenyl groupsattached to silicon atoms in a molecule. The content of alkenyl groupsis preferably 0.001 to 5 mol %, especially 0.01 to 0.5 mol % based onthe R¹ groups. With respect to the molecular structure, theorganopolysiloxane is generally of straight chain although it may havein part branched siloxane skeletons. The degree of polymerization is inthe range which is known in the art as the range of organopolysiloxanegum, and typically in the range of about 3,000 to 20,000, especiallyabout 5,000 to 10,000. The organopolysiloxane is preferably end-cappedwith a triorganosilyl group such as vinyldimethylsilyl,divinylmethylsilyl or trivinylsilyl.

[0022] Component (B) is an inorganic filler which may be selected fromconventional fillers used in silicone rubber. Useful fillers includefinely divided silica, quartz flour, magnesium oxide, diatomaceousearth, calcium carbonate, and carbon black, with the finely dividedsilica being preferred. Silica powder has a specific surface area asmeasured by the BET method of at least 50 m²/g, especially 100 to 400m²/g. Wet silica and fumed silica (dry silica) are especially preferred.For wire coating compositions and sponge compositions, fumed silica ispreferred among others. Reinforcing silica which can be used herein iscommercially available under the trade name of Aerosil 130, 200, 300 and380 from Nippon Aerosil K.K., Cab-O-Sil MS-5, MS-7, HS-5 and HS-7 fromCabot Corp., Santocel FRC and CS from Monsanto Co., and Nipsil VN-3 fromNippon Silica Industry K.K. These fillers may be used with or withoutsurface treatment with organopolysiloxanes, organopolysilazanes,chlorosilanes or alkoxysilanes. The amount of the filler used herein isnot critical and preferably ranges from about 5 to 200 parts by weightper 100 parts by weight of the organopolysiloxane (A). An appropriateamount of finely divided silica is about 5 to 100 parts, more preferablyabout 10 to 90 parts, and especially about 30 to 80 parts by weight per100 parts by weight of the organopolysiloxane (A).

[0023] Component (C) is an organic peroxide of the structural formula(2).

[0024] In formula (2), R² which may be the same or different isindependently selected from hydrogen and alkyl groups, preferably of 1to 8 carbon atoms, more preferably 1 to 4 carbon atoms, such as methyl,ethyl, propyl and butyl. R³ is selected from alkylene groups, preferablyof 1 to 12 carbon atoms, more preferably 2 to 8 carbon atoms, such asmethylene, ethylene, propylene, butylene and hexylene. The letter n isan integer of 1 to 3.

[0025] Specifically the organic peroxides of structural formula (2)include 1,6-bis(p-toluoylperoxycarbonyloxy)-hexane of the followingstructural formula (3), 1,6-bis(benzoylperoxycarbonyloxy)hexane of thefollowing structural formula (4),1,6-bis(p-toluoylperoxycarbonyloxy)butane, and1,6-bis(2,4-dimethylbenzoylperoxy-carbonyloxy)hexane.

[0026] In order that the organic peroxide of formula (2) be admixed witha compound comprised of the organopolysiloxane (A) and the filler (B) ina safe manner, the organic peroxide may take the form of a paste in aninert carrier, preferably a diorganopolysiloxane compatible with thecompound such as polydimethylsiloxane, prior to use. An appropriateamount of the organic peroxide of formula (2) is about 0.1 to 10 parts,and especially about 0.2 to 5 parts by weight per 100 parts by weight ofthe organopolysiloxane (A).

[0027] By blending (E) an organic blowing agent, the silicone rubbercomposition of the invention can be formulated as a silicone rubbersponge composition.

[0028] Examples of the organic blowing agent (E) include organic azocompounds such as azobisisobutyronitrile,2,2′-azobis-2-methylbutyronitrile,2,2′-azobis-2,4-dimethylvaleronitrile,1,1′-azobis(1-acetoxy-1-phenylethane), and azodicarbonamide; nitrosocompounds such as dinitropentamethylene tetramine; and hydrazinederivatives such as 4,4′-oxybis(benzenesulfonylhydrazide) andp-toluenesulfonyl hydrazine. Of these, the azo compounds are preferred,with azobisisobutyronitrile being especially preferred.

[0029] An appropriate amount of the blowing agent (E) blended is about0.01 to 50 parts, and especially about 0.5 to 10 parts by weight per 100parts by weight of the organopolysiloxane (A). Less than 0.01 part byweight of the blowing agent is ineffective for the blowing purposewhereas more than 50 parts by weight of the blowing agent may createlarge and uneven cells and fail to form a skin layer.

[0030] Another organic peroxide (D) may be blended herein insofar as theadvantages of the invention are not impaired. Examples of the otherorganic peroxide are dialkyl organic peroxides and peroxy ester organicperoxides.

[0031] Especially in the silicone rubber sponge composition, a dialkylperoxide and/or peroxy ester is preferably used in combination for thepurposes of increasing a blowing magnification and creating more uniformcells. Examples include dicumyl peroxide,2,5-dimethyl-bis(t-butylperoxyhexane),2,5-dimethyl-2,5-bis(t-butylperoxy)hexyne, t-butylperoxy benzoate, and1,6-bis(t-butylperoxycarbonyloxy)hexane.

[0032] An appropriate amount of the dialkyl peroxide and peroxy esterblended is about 0.1 to 10 parts, and especially about 0.2 to 5 parts byweight per 100 parts by weight of the organopolysiloxane (A).

[0033] In the silicone rubber sponge composition, an addition typecrosslinking agent may be additionally used for the purposes of creatingmore uniform cells and improving the compression set of the sponge.Well-known addition type crosslinking agents are useful, andcombinations of platinum group catalysts with organohydrogenpolysiloxanehaving at least two SiH groups in a molecule are often used.

[0034] The platinum group catalyst may be any of well-known additionreaction catalysts, for example, metal elements of the platinum groupand compounds thereof. Examples include microparticulate platinum metaladsorbed on supports such as silica, alumina and silica gel, platinicchloride, chloroplatinic acid, complexes of chloroplatinic acidhexahydrate with olefins or divinyldimethylpolysiloxane, alcoholsolutions of chloroplatinic acid hexahydrate, palladium catalysts, andrhodium catalysts. The catalyst is used in a catalytic amount, andtypically in an amount of about 1 to 2,000 ppm, preferably about 10 to500 ppm of platinum group metal. Less than 1 ppm fail to fully promotecrosslinking reaction and result in undercure whereas more than 2,000ppm affects reactivity little further and is uneconomical.

[0035] The organohydrogenpolysiloxane having at least two SiH groups ina molecule may be straight, cyclic or branched. There may be used any oforganohydrogenpolysiloxanes well known as the curing agent in additionreaction curing type silicone rubber compositions although anorganohydrogenpolysiloxane of the following average compositionalformula (5) is often useful.

R⁴ _(x)H_(y)SiO_((4−x−y)/2)  (5)

[0036] Herein R⁴ is a substituted or unsubstituted monovalenthydrocarbon group as defined for R¹, preferably of 1 to 12 carbon atoms,especially 1 to 8 carbon atoms, for example, alkyl, alkenyl, aryl andaralkyl groups, and halogen- and cyano-substituted groups thereof. Theletter x is a positive number of from 1 to 2.2, y is a positive numberof from 0.002 to 1, and the sum of x+y is from 1.002 to 3. Theorganohydrogenpolysiloxane has in a molecule at least two, preferably atleast three SiH groups, which may be positioned at ends or midway of themolecular chain. Its viscosity is preferably less than about 300centistokes at 25° C. An appropriate amount oforganohydrogenpolysiloxane blended is about 0.01 to 10 parts by weightper 100 parts by weight of the organopolysiloxane (A). More preferablythe organohydrogenpolysiloxane is added in such amounts that the ratioof silicon atom-bonded hydrogen atoms to alkenyl groups in component (A)may be from 0.5 to 10, especially from 1 to 4. A hydrogen/alkenyl ratioof less than 0.5 may lead to insufficient crosslinking and insufficientmechanical strength whereas a hydrogen/alkenyl ratio of more than 10 maycause deterioration of cured physical properties and especially,substantial losses of heat resistance and compression set.

[0037] In the silicone rubber composition and silicone rubber spongecomposition according to the invention, flame retardants, fireresistance modifiers, heat resistance modifiers, and coloring agents maybe blended, if necessary, as well as dispersants such as alkoxysilanes,carbon functional silanes, and silanol group-bearing low molecularweight siloxanes.

[0038] Where the silicone rubber composition is used for the coating ofan electrically insulated wire, an adhesion inhibitor and/or heatresistance modifier is preferably blended therein. These additives arecustomarily used in conventional wire-coating silicone rubbercompositions and effective for the purposes of preventing close adhesionto the conductor such as a copper conductor or tin-plated copperconductor and improving the heat resistance, respectively.

[0039] Examples of the adhesion inhibitor include paraffins andhydrocarbon resins such as paraffin wax and microcrystalline wax; fattyacids and metal salts thereof such as stearic acid, zinc stearate andcalcium stearate; fatty acid amides such as stearamide andmethylenebis-stearamide; fatty acid esters such as n-butyl stearate; andester waxes. Desirably, the adhesion inhibitor is added in an amount ofabout 0.001 to 5 parts, especially about 0.05 to 1 part by weight per100 parts by weight of the organopolysiloxane (A). Less than 0.001 partof the adhesion inhibitor may allow the composition to closely bond tothe conductor and be ineffective for its purpose. More than 5 parts maysometimes adversely affect the physical properties of rubber.

[0040] Examples of the heat resistance modifier include metal oxidessuch as iron oxide, cerium oxide, zinc oxide, and titanium oxide; ceriumsilanolate and cerium fatty acid salts. Desirably, the heat resistancemodifier is added in an amount of about 0.01 to 5 parts, especiallyabout 0.2 to 3 parts by weight per 100 parts by weight of theorganopolysiloxane (A). Less than 0.01 part of the heat resistancemodifier may be ineffective for its purpose whereas more than 5 partsmay achieve no further improvement in heat resistance.

[0041] The composition of the invention may be prepared by uniformlymixing the above essential and optional components in a rubber mixersuch as a twin-roll mill, Banbury mixer or dough mixer (kneader),followed by optional heat treatment.

[0042] With respect to the molding and vulcanization of the composition,any desired vulcanizing method may be used as long as it can apply asufficient amount of heat to decompose the curing agent. Suitablemolding methods include continuous atmospheric hot air vulcanization inline with extrusion molding, press vulcanization, and injectionvulcanization and are not limited thereto. In the practice of theinvention, atmospheric hot air vulcanization is advantageously employed.Appropriate conditions therefor include a temperature of about 100 to500° C., especially about 150 to 400° C. and a time of about severalseconds to 1 hour, especially about 10 seconds to 30 minutes. Ifnecessary, this is followed by secondary vulcanization at about 150 to250° C. for about 1 to 10 hours.

[0043] A silicone rubber-covered wire is manufactured by continuouslyfeeding the composition around a wire, typically a copper wire ortin-plated copper wire through an extruder, followed by atmospheric hotair vulcanization in a hot air furnace. If the vulcanization temperatureis below 200° C., there is a possibility that vulcanization be retardedand micro-voids form. A vulcanization temperature above 550° C. maycause over-vulcanization and hence, surface cracks. For this reason, thetemperature of the heating furnace is preferably set in the range ofabout 200 to 550° C., especially about 250 to 500° C., and the residencetime within the furnace is preferably set in the range of about 1 to 300seconds, especially about 5 to 50 seconds. The hot air vulcanizationfurnace may be combined with a radiation heating furnace such as a UHFvulcanization furnace or electron beam vulcanization furnace.

[0044] There has been described a silicone rubber composition which hasa high vulcanization rate and hygienic safety, and can be extrusionvulcanized in a continuous atmospheric hot air vulcanization manner intoa cured silicone rubber having satisfactory physical properties withoutgenerating voids or surface tack. When the composition is applied to awire to construct a silicone rubber-covered wire, the coating hasimproved withstand voltage properties because of the absence ofmicro-voids and requires no dusting powder because of the eliminatedsurface tack. The coating allows for smooth cut/stripping required onthe processing of wire terminal ends.

[0045] Since halogen-free organic peroxides are used, the siliconerubber sponge composition is safe and effectively expandable and can becured, without a need for a long term of post curing, into a siliconerubber sponge having uniform fine cells and a skin layer with a smooth,tack-free surface.

EXAMPLE

[0046] Examples of the invention are given below by way of illustrationand not by way of limitation.

Example 1

[0047] Compound 1 was prepared by milling 100 parts by weight of anorganopolysiloxane consisting of 99.850 mol % of dimethylsiloxane units,0.125 mol % of methylvinylsiloxane units and 0.025 mol % ofdimethylvinylsiloxane units and having an average degree ofpolymerization of about 8000, 45 parts by weight of fumed silica havinga specific surface area of 200 m²/g (Aerosil 200 by Nippon AerosilK.K.), and 10 parts by weight of both end silanol group-blockeddimethylpolysiloxane having an average degree of polymerization of 13and a viscosity of 15 centistokes at 25° C. as the dispersant in akneader, and heat treating the mixture at 180° C. for 3 hours.

[0048] Using a two-roll mill, 100 parts by weight of Compound 1 wasblended with 1.0 part by weight of a 50% silicone oil paste of1,6-bis(p-toluoylperoxycarbonyloxy)hexane as the curing agent. A sheetof 2 mm thick was formed therefrom and heat treated at 250° C. for 10minutes.

[0049] The silicone rubber sheet was examined for surface tack andexpanded state in cross section, with the results shown in Table 1.

Comparative Example 1

[0050] Using a two-roll mill, 100 parts by weight of Compound 1 wasblended with 1.0 part by weight of a 50% silicone oil paste ofbis(p-methylbenzoyl)peroxide as the curing agent. A sheet of 2 mm thickwas formed therefrom and heat treated at 250° C. for 10 minutes.

[0051] The silicone rubber sheet was examined for surface tack andexpanded state in cross section, with the results shown in Table 1.

Comparative Example 2

[0052] Using a two-roll mill, 100 parts by weight of Compound 1 wasblended with 0.5 part by weight of1,3-bis(t-butylperoxycarbonyloxy)-2,2-dimethylpropane as the curingagent. A sheet of 2 mm thick was formed therefrom and heat treated at250° C. for 10 minutes.

[0053] The silicone rubber sheet was examined for surface tack andexpanded state in cross section, with the results shown in Table 1.

Comparative Example 3

[0054] Using a two-roll mill, 100 parts by weight of Compound 1 wasblended with 1.0 part by weight of a 50% silicone oil paste ofbis(2,4-dichlorobenzoyl)peroxide as the curing agent. A sheet of 2 mmthick was formed therefrom and heat treated at 250° C. for 10 minutes.

[0055] The silicone rubber sheet was examined for surface tack andexpanded state in cross section, with the results shown in Table 1.TABLE 1 Curing agent Tack Expanded E11,6-bis(p-toluoylperoxy-carbonyloxy)hexane No No CE1bis(p-methylbenzoyl)peroxide No Yes CE21,3-bis(t-butylperoxycarbonyloxy)-2,2- No Yes dimethylpropane CE3bis(2,4-dichlorobenzoyl)peroxide No No

Example 2

[0056] Using a two-roll mill, 100 parts by weight of Compound 1 wasblended with 1.0 part by weight of a 50% silicone oil paste of1,6-bis(p-toluoylperoxycarbonyloxy)hexane as the curing agent. This waspress cured at 120° C. for 10 minutes and post cured at 200° C. for 4hours. The physical properties of the sample were measured, with theresults shown in Table 2.

Comparative Example 4

[0057] Using a two-roll mill, 100 parts by weight of Compound 1 wasblended with 1.0 part by weight of a 50% silicone oil paste ofbis(p-methylbenzoyl)peroxide as the curing agent. This was press curedat 120° C. for 10 minutes and post cured at 200° C. for 4 hours. Thephysical properties of the sample were measured, with the results shownin Table 2.

Comparative Example 5

[0058] Using a two-roll mill, 100 parts by weight of Compound 1 wasblended with 0.5 part by weight of1,3-bis(t-butylperoxycarbonyloxy)-2,2-dimethylpropane as the curingagent. This was press cured at 120° C. for 10 minutes and post cured at200° C. for 4 hours. The physical properties of the sample weremeasured, with the results shown in Table 2.

Comparative Example 6

[0059] Using a two-roll mill, 100 parts by weight of Compound 1 wasblended with 1.0 part by weight of a 50% silicone oil paste ofbis(2,4-dichlorobenzoyl)peroxide as the curing agent. This was presscured at 120° C. for 10 minutes and post cured at 200° C. for 4 hours.The physical properties of the sample were measured, with the resultsshown in Table 2. TABLE 2 Tensile Elongation Hardness strength at breakCuring agent (Type A) (MPa) (%) E2 1,6-bis(p- 60 10.1 520toluoylperoxycarbonyloxy) hexane CE4 bis(p-methylbenzoyl)peroxide 61 9.4480 CE5 1,3-bis(t- 58 9.5 530 butylperoxycarbonyloxy)-2,2-dimethylpropane CE6 bis(2,4- 60 9.8 530 dichlorobenzoyl)peroxide

Example 3

[0060] Using a two-roll mill, 100 parts by weight of Compound 1 wasblended with 1.0 part by weight of a 50% silicone oil paste of1,6-bis(p-toluoylperoxycarbonyloxy)hexane as the curing agent. Using amoving die rheometer, the vulcanization rates T10 and T90 at 120° C. ofthe sample were measured. It is noted that T10 and T90 are times takenuntil the torque reaches 10% and 90% of the maximum torque,respectively. The results are shown in Table 3.

Comparative Example 7

[0061] Using a two-roll mill, 100 parts by weight of Compound 1 wasblended with 1.0 part by weight of a 50% silicone oil paste ofbis(p-methylbenzoyl)peroxide as the curing agent. Using the moving dierheometer, the vulcanization rates T10 and T90 at 120° C. of the samplewere measured. The results are shown in Table 3.

Comparative Example 8

[0062] Using a two-roll mill, 100 parts by weight of Compound 1 wasblended with 0.5 part by weight of1,3-bis(t-butylperoxycarbonyloxy)-2,2-dimethylpropane as the curingagent. Using the moving die rheometer, the vulcanization rates T10 andT90 at 120° C. of the sample were measured. The results are shown inTable 3.

Comparative Example 9

[0063] Using a two-roll mill, 100 parts by weight of Compound 1 wasblended with 1.0 part by weight of a 50% silicone oil paste ofbis(2,4-dichlorobenzoyl)peroxide as the curing agent. Using the movingdie rheometer, the vulcanization rates T10 and T90 at 120° C. of thesample were measured. The results are shown in Table 3. TABLE 3 T10 T90Curing agent (min) (min) E3 1,6-bis(p-toluoylperoxy-carbonyloxy)hexane0.25 0.63 CE7 bis(p-methylbenzoyl)peroxide 0.55 2.23 CE81,3-bis(t-butylperoxycarbonyloxy)-2,2- 1.72 8.53 dimethylpropane CE9bis(2,4-dichlorobenzoyl)peroxide 0.32 0.70

Example 4

[0064] Hundred parts by weight of a heat-curable silicone rubbercompound (KE904FU by Shin-Etsu Chemical Co., Ltd.) loaded with about 25%by weight of reinforcing finely divided silica powder was blended with 2parts by weight of azobisisobutyronitrile and 1.0 part by weight of a50% silicone oil paste of 1,6-bis(p-toluoylperoxycarbonyloxy)-hexane.Using a two-roll mill, a sheet of 2 mm thick was formed therefrom. Thesheet was heated at 250° C. for 10 minutes. The sponge thus obtained hada blowing magnification of 500%, no coloring, uniform cells with a sizeof less than 1 mm, and a smooth, tack-free surface. The sponge exhibiteda compression set of 20% after 50% compression at 180° C. and 22 hours.

Example 5

[0065] Hundred parts by weight of a heat-curable silicone rubbercompound (KE904FU by Shin-Etsu Chemical Co., Ltd.) was blended with 2parts by weight of azobisisobutyronitrile, 1.0 part by weight of a 50%silicone oil paste of 1,6-bis(p-toluoylperoxycarbonyloxy)hexane, and 0.6part by weight of dicumyl peroxide. Using a two-roll mill, a sheet of 2mm thick was formed therefrom. The sheet was heated at 250° C. for 10minutes. The sponge thus obtained had a blowing magnification of 600%,no coloring, uniform cells with a size of less than 1 mm, and a smooth,tack-free surface. The sponge exhibited a compression set of 15% after50% compression at 180° C. and 22 hours.

Example 6

[0066] Hundred parts by weight of a heat-curable silicone rubbercompound (KE904FU by Shin-Etsu Chemical Co., Ltd.) was blended with 2parts by weight of azobisisobutyronitrile, 1.0 part by weight of a 50%silicone oil paste of 1,6-bis(p-toluoylperoxycarbonyloxy)hexane, 0.5part by weight of a polyvinylsiloxane complex of a platinum compound,0.05 part by weight of 1-ethynylcyclohexan-1-ol, and 1.0 part by weightof a both end trimethylsiloxy group-blockeddimethylsiloxane/methylhydrogensiloxane copolymer (Si—H 0.007 mol/g).Using a two-roll mill, a sheet of 2 mm thick was formed therefrom. Thesheet was heated at 250° C. for 10 minutes. The sponge thus obtained hada blowing magnification of 450%, no coloring, uniform cells with a sizeof less than 1 mm, and a smooth, tack-free surface. The sponge exhibiteda compression set of 10% after 50% compression at 180° C. and 22 hours.

Example 7

[0067] Hundred parts by weight of a heat-curable silicone rubbercompound (KE904FU by Shin-Etsu Chemical Co., Ltd.) was blended with 2parts by weight of azobisisobutyronitrile, 1.0 part by weight of a 50%silicone oil paste of 1,6-bis(p-toluoylperoxycarbonyloxy)hexane, and 0.6part by weight of dicumyl peroxide. Using a two-roll mill, a sheet of 2mm thick was formed therefrom. The sheet was heated at 250° C. for 10minutes. The sponge thus obtained had a blowing magnification of 600%,no coloring, uniform cells with a size of less than 1 mm, and a smooth,tack-free surface. The sponge exhibited a compression set of 20% after50% compression at 180° C. and 22 hours.

Comparative Example 10

[0068] Hundred parts by weight of a heat-curable silicone rubbercompound (KE904FU by Shin-Etsu Chemical Co., Ltd.) was blended with 2parts by weight of azobisisobutyronitrile and 0.6 part by weight ofdicumyl peroxide. Using a two-roll mill, a sheet of 2 mm thick wasformed therefrom. The sheet was heated at 250° C. for 10 minutes. Thesponge thus obtained had a blowing magnification of 250% and non-uniformcells with a size of more than 1 mm.

Comparative Example 11

[0069] Hundred parts by weight of a heat-curable silicone rubbercompound (KE904FU by Shin-Etsu Chemical Co., Ltd.) was blended with 2parts by weight of azobisisobutyronitrile, 0.5 part by weight of apolyvinylsiloxane complex of a platinum compound, 0.05 part by weight of1-ethynylcyclohexan-1-ol, and 1.0 part by weight of a both endtrimethylsiloxy group-blocked dimethylsiloxane/methyl-hydrogensiloxanecopolymer (Si—H 0.007 mol/g). Using a two-roll mill, a sheet of 2 mmthick was formed therefrom. The sheet was heated at 250° C. for 10minutes. The sponge thus obtained had a blowing magnification of 200%and non-uniform cells with a size of more than 1 mm.

Example 8

[0070] Compound 2 was prepared by milling 100 parts by weight of anorganopolysiloxane consisting of 99.825 mol % of dimethylsiloxane units,0.15 mol % of methylvinylsiloxane units and 0.025 mol % ofdimethylvinylsiloxane units and having an average degree ofpolymerization of about 8000, 45 parts by weight of fumed silica havinga specific surface area of 200 m²/g (Aerosil 200 by Nippon AerosilK.K.), and 10 parts by weight of both end silanol group-blockeddimethylpolysiloxane having an average degree of polymerization of 13and a viscosity of 15 centistokes at 25° C. as the dispersant in akneader, and heat treating the mixture at 180° C. for 3 hours.

[0071] Hundred parts by weight of Compound 2 was blended with 1.5 partsby weight of azobisisobutyronitrile as the blowing agent, 1.0 part byweight of a 50% silicone oil paste of1,6-bis(p-toluoylperoxycarbonyloxy)hexane and 0.6 part by weight ofdicumyl peroxide. Using a two-roll mill, a sheet of 2 mm thick wasformed therefrom, followed by heat treatment at 250° C. for 10 minutes.The sponge thus obtained had a blowing magnification of 400%, nocoloring, uniform cells with a size of about 200 μm, and a smooth,tack-free surface. The sponge exhibited a compression set of 10% after50% compression at 180° C. and 22 hours.

Example 9

[0072] Hundred parts by weight of Compound 2 was blended with 1.5 partsby weight of azobisisobutyronitrile as the blowing agent, 1.0 part byweight of a 50% silicone oil paste of1,6-bis(p-toluoylperoxycarbonyloxy)butane and 0.6 part by weight ofdicumyl peroxide. Using a two-roll mill, a sheet of 2 mm thick wasformed therefrom, followed by heat treatment at 250° C. for 10 minutes.The sponge thus obtained had a blowing magnification of 450%, nocoloring, uniform cells with a size of about 300 μm, and a smooth,tack-free surface. The sponge exhibited a compression set of 15% after50% compression at 180° C. and 22 hours.

Examples 10-11 and Comparative Examples 12-13

[0073] Compound 3 was prepared by milling 100 parts by weight of anorganopolysiloxane consisting of 99.850 mol % of dimethylsiloxane units,0.125 mol % of methylvinylsiloxane units and 0.025 mol % ofdimethylvinylsiloxane units and having an average degree ofpolymerization of about 8000, 40 parts by weight of fumed silica havinga specific surface area of 200 m²/g (Aerosil 200 by Nippon AerosilK.K.), 30 parts by weight of diatomaceous earth Celite SF (JohnManville), and 10 parts by weight of both end silanol group-blockeddimethylpolysiloxane having an average degree of polymerization of 13and a viscosity of 15 centistokes at 25° C. as the dispersant in akneader, and heat treating the mixture at 180° C. for 3 hours.

[0074] Using a two-roll mill, 100 parts by weight of Compound 3 wasblended with 1.0 part by weight of a 50% silicone oil paste of1,6-bis(p-toluoylperoxycarbonyloxy)hexane as the curing agent. This isdesignated silicone rubber composition 1.

[0075] Using a two-roll mill, 100 parts by weight of Compound 3 wasblended with 1.0 part by weight of a 50% silicone oil paste of1,6-bis(benzoylperoxycarbonyloxy)hexane as the curing agent. This isdesignated silicone rubber composition 2.

[0076] Using a two-roll mill, 100 parts by weight of Compound 3 wasblended with 1.0 part by weight of a 50% silicone oil paste ofbis(o-methylbenzoyl)peroxide as the curing agent. This is designatedsilicone rubber composition 3.

[0077] Using a two-roll mill, 100 parts by weight of Compound 3 wasblended with 1.0 part by weight of a 50% silicone oil paste ofbis(p-methylbenzoyl)peroxide as the curing agent. This is designatedsilicone rubber composition 4.

[0078] Using a 40-mm diameter extruder, each of silicone rubbercompositions 1 to 4 was extrusion molded into a rod having a diameter of6 mm. The rod was subjected to atmospheric hot air vulcanization at 380°C. for 13 seconds and the cured rod was examined for micro-voids by thetest procedure to be described below. Separately, using a 40-mm diameterextruder, each of silicone rubber compositions 1 to 4 was molded arounda tin-plated soft copper conductor of diameter 0.6 mm, forming a coveredwire having an outer diameter of 1.2 mm. The coating was subjected toatmospheric hot air vulcanization at 450° C. for 30 seconds and thecovered wire was examined by the withstand voltage test to be describedbelow. The results are shown in Table 4.

[0079] Using a disk rheometer, the silicone rubber compositions 1 to 4were examined for vulcanizing property according to the measurementprocedure to be described below. The results are shown in Table 5.

[0080] Measurement Methods

[0081] Vulcanizing Property

[0082] Using a disk rheometer (Monsanto Co.), a torque was measured at120° C. and a deflection angle of ±3°.

[0083] Micro-Voids

[0084] A silicone rubber composition to be tested was extrusion moldedthrough a 40-mm diameter extruder into a rod having a diameter of 6 mm,which was passed through a vulcanizing furnace at 380° C. for aresidence time of 13 seconds for effecting atmospheric hot airvulcanization. A cross section of the cured rod was observed under anoptical microscope with a magnifying power of X100 to inspectmicro-voids.

[0085] Breakdown Strength

[0086] Using a 40-mm diameter extruder, a silicone rubber composition tobe tested was molded around a tin-plated soft copper conductor ofdiameter 0.6 mm, forming a covered wire having an outer diameter of 1.2mm. This was subjected to atmospheric hot air vulcanization at 450° C.for 30 seconds. In this way, ten samples were prepared from eachcomposition. The samples were examined by a withstand voltage test inwater according to JIS C-3004.

[0087] Cut/Stripping

[0088] Using a 40-mm diameter extruder, a silicone rubber composition tobe tested was molded around a tin-plated soft copper conductor ofdiameter 0.6 mm, forming a covered wire having an outer diameter of 1.2mm. This was subjected to atmospheric hot air vulcanization at 450° C.for 30 seconds. The covered wire was examined for ease of stripping ofthe rubber coating. The jig used for examining cut/stripping was aspecial jig comprising a radio pliers-shaped clamp combined with acutting edge for cutting off the rubber coating. It was visuallyobserved how much the rubber was left on the conductor surface wherestripped. TABLE 4 Micro-voids Breakdown strength (kV/mm) E10(Composition 1) nil 25 E11 (Composition 2) nil 25 CE12 (Composition 3)found 22 CE13 (Composition 4) found 21

[0089] The samples of the silicone rubber compositions within the scopeof the invention exhibited a higher withstand voltage than thecomparative examples. TABLE 5 T10 T90 (induction time, (appropriatecrosslinking min) time, min) E10 (Composition 1) 0.26 0.76 E11(Composition 2) 0.29 0.76 CE12 (Composition 3) 0.42 1.72 CE13(Composition 4) 0.59 2.58

Examples 12-13 and Comparative Examples 14-15

[0090] Compound 4 was prepared by milling 100 parts by weight of anorganopolysiloxane consisting of 99.850 mol % of dimethylsiloxane units,0.125 mol % of methylvinylsiloxane units and 0.025 mol % ofdimethylvinylsiloxane units and having an average degree ofpolymerization of about 8000, 50 parts by weight of fumed silica havinga specific surface area of 200 m²/g treated with dichlorodimethylsilane(Aerosil R-972 by Nippon Aerosil K.K.), and 3 parts by weight of bothend silanol group-blocked dimethylpolysiloxane having an average degreeof polymerization of 13 and a viscosity of 15 centistokes at 25° C. asthe dispersant in a kneader, heat treating the mixture at 180° C. for 3hours, and further blending therein 0.1 part by weight of zinc stearateand 1.0 part by weight of cerium oxide.

[0091] Using a two-roll mill, 100 parts by weight of Compound 4 wasblended with 1.0 part by weight of a 50% silicone oil paste of1,6-bis(p-toluoylperoxycarbonyloxy)hexane as the curing agent. This isdesignated silicone rubber composition 5.

[0092] Using a two-roll mill, 100 parts by weight of Compound 4 wasblended with 1.0 part by weight of a 50% silicone oil paste of1,6-bis(benzoylperoxycarbonyloxy)hexane as the curing agent. This isdesignated silicone rubber composition 6.

[0093] Using a two-roll mill, 100 parts by weight of Compound 4 wasblended with 1.0 part by weight of a 50% silicone oil paste ofbis(o-methylbenzoyl)peroxide as the curing agent. This is designatedsilicone rubber composition 7.

[0094] Using a two-roll mill, 100 parts by weight of Compound 4 wasblended with 1.0 part by weight of a 50% silicone oil paste ofbis(p-methylbenzoyl)peroxide as the curing agent. This is designatedsilicone rubber composition 8.

[0095] Using a 40-mm diameter extruder, each of silicone rubbercompositions 5 to 8 was extrusion molded into a rod having a diameter of6 mm. The rod was subjected to atmospheric hot air vulcanization at 380°C. for 13 seconds and the cured rod was examined for micro-voids by theabove test procedure. Separately, using a 40-mm diameter extruder, eachof silicone rubber compositions 5 to 8 was molded around a tin-platedsoft copper conductor of diameter 0.6 mm, forming a covered wire havingan outer diameter of 1.2 mm. The coating was subjected to atmospherichot air vulcanization at 450° C. for 30 seconds, and the covered wirewas examined by the above withstand voltage test and cut/stripping test.The results are shown in Table 6. TABLE 6 Breakdown strength Cut/Micro-voids (kV/mm) stripping E12 (Composition 5) nil 27 good E13(Composition 6) nil 27 good CE14 (Composition 7) found 24 fairly stuckCE15 (Composition 8) found 22 stuck

[0096] The samples of the silicone rubber compositions within the scopeof the invention exhibited a higher withstand voltage and bettercut/stripping than the comparative examples.

[0097] Japanese Patent Application Nos. 2000-012899 and 2000-239304 areincorporated herein by reference.

[0098] Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A silicone rubber composition comprising (A) an organopolysiloxane ofthe average compositional formula (1): R¹ _(a)SiO_((4−a)/2)  (1) whereinR¹ is a substituted or unsubstituted monovalent hydrocarbon group and“a” is a positive number of 1.8 to 2.3, (B) an inorganic filler, and (C)an organic peroxide of the structural formula (2):

wherein R² is independently hydrogen or alkyl, R³ is alkylene, and n isan integer of 1 to
 3. 2. The composition of claim 1 wherein theinorganic filler (B) is finely divided silica.
 3. The composition ofclaim 2 wherein the inorganic filler (B) is fumed silica.
 4. Thecomposition of claim 1 wherein the organic peroxide (C) is1,6-bis(p-toluoylperoxycarbonyloxy)hexane of the following structuralformula (3) or 1,6-bis(benzoylperoxycarbonyloxy)hexane of the followingstructural formula (4):


5. The composition of claim 1 further comprising (D) a dialkyl organicperoxide or peroxy ester organic peroxide or a mixture thereof.
 6. Thecomposition of claim 1 for covering an electrical wire.
 7. A siliconerubber-covered wire comprising a wire in the form of a copper wire ortin-plated copper wire around which the silicone rubber composition ofclaim 6 has been extrusion molded.
 8. A silicone rubber spongecomposition comprising the silicone rubber composition of claim 1 , and(E) an organic blowing agent.